The mechanism of water pollutant photodegradation by mixed and core–shell WO3/TiO2 nanocomposites

Environmental pollution is one of the biggest concerns in the world today, and solar energy-driven photocatalysis is a promising method for decomposing pollutants in aqueous systems. In this study, the photocatalytic efficiency and catalytic mechanism of WO3-loaded TiO2 nanocomposites of various structures were analyzed. The nanocomposites were synthesized via sol–gel reactions using mixtures of precursors at various ratios (5%, 8%, and 10 wt% WO3 in the nanocomposites) and via core–shell approaches (TiO2@WO3 and WO3@TiO2 in a 9 : 1 ratio of TiO2 : WO3). After calcination at 450 °C, the nanocomposites were characterized and used as photocatalysts. The kinetics of photocatalysis with these nanocomposites for the degradation of methylene blue (MB+) and methyl orange (MO−) under UV light (365 nm) were analyzed as pseudo-first-order reactions. The decomposition rate of MB+ was much higher than that of MO−, and the adsorption behavior of the dyes in the dark suggested that the negatively charged surface of WO3 played an important role in adsorbing the cationic dye. Scavengers were used to quench the active species (superoxide, hole, and hydroxyl radicals), and the results indicated that hydroxyl radicals were the most active species; however, the active species were generated more evenly on the mixed surfaces of WO3 and TiO2 than on the core–shell structures. This finding shows that the photoreaction mechanisms could be controlled through adjustments to the nanocomposite structure. These results can guide the design and preparation of photocatalysts with improved and controlled activities for environmental remediation.


Introduction
Increasing wastewater discharges from various sources pose enormous environmental challenges worldwide. 1 Due to rapid industrial growth, the environment has become highly contaminated with various organic and inorganic pollutants. 2,3 Dyes are common hazardous organic contaminants in wastewater. 4 They also impart color to the water and can produce harmful byproducts through chemical reactions. 5 The longterm consumption of water containing dyes could harm the liver, central nervous, and digestive systems of humans. 6 For this reason, many researchers are engaged in developing techniques to eliminate organic dyes from water systems.
Recently, various techniques, including using a green biochar/iron oxide composite, 7 coated membranes, 8 surfactant-modied biomass, 9 coagulation, 10 modied magnetic nanosorbents, 11,12 hydrochar adsorption, 13,14 and photocatalytic degradation, 15,16 have been applied to remove both cationic and anionic dyes from water. Among these methods, semiconductor-based photocatalysis is thought to be the most promising method because it can convert a broad range of organic contaminants into less toxic compounds, including CO 2 , and H 2 O, without the use of expensive oxidants. With the aim of developing effective photocatalysts, various semiconductors have been examined individually or in combination with other materials. The modication of photocatalyst surfaces with other materials can improve the efficiency of photocatalysis. 17 Combining semiconductors with metals can enhance charge separation. [18][19][20][21] Elemental doping and combining different semiconductors 22,23 can change the band gap of the resulting materials and induce charge separation. Among the photocatalysts, TiO 2 has been widely investigated as a typical semiconductor photocatalyst 24,25 due to its high photocatalytic activity, low price, physicochemical stability, nontoxicity, and environmental friendliness. 26 Despite these advantages, the wide bandgap of TiO 2 (3.20-3.35 eV) limits the use of light to the UV range, shows rapid charge recombination and has limited efficiency. 27 To address these limitations, doping and combining TiO 2 photocatalysts with narrow bandgap semiconductors are viable options. Semiconductors such as MoO 3 , 28 Ag 2 CO 3 , 29 ZnO, 30 and WO 3 (ref. 27) have been coupled with TiO 2 to improve its photocatalytic efficiency under UV light. Among them, WO 3 has attracted considerable amounts of attention due to its ability to absorb visible light (typically wavelengths <500 nm (ref. 31 and 32) and extended to >500 nm by the effects of oxygen vacancies 32 ) WO 3 is also stable in oxidative and acidic environments and has low cost and low toxicity. 33 The crystal ionic radius of W 6+ is close to that of T 4+ ; therefore, W 6+ can be easily introduced into the lattice of titania to replace Ti 4+ and form W-O-Ti links, or it can be positioned at interstitial locations, 34,35 which effectively induces lattice defects and increases the surface area of WO 3 -coupled TiO 2 . Moreover, WO 3 -coupled TiO 2 shows enhanced O 2 chemisorption on its surfaces, 36 and this adsorbed oxygen improves charge separation. Thus, WO 3 -coupled TiO 2 has emerged as a promising adsorbent and catalyst. However, based on the preparation methods and the nature of pollutants to be decomposed, different behaviors have been reported. 34,[36][37][38][39][40] Some studies have stated that WO 3 doping boosted TiO 2 photocatalytic activity, whereas others reported that it had the reverse effect. Various factors, such as the nature of the dopants and their concentrations, the nature of pollutants, the intensity of light and irradiation time, dissolved oxygen concentration, reaction temperature, pH, surface area, the quantity of catalyst, and the surface morphology of the catalysts, 41 are now considered to have an impact on photocatalytic decomposition. For effective photocatalysts, WO 3 /TiO 2 core-shell nanorods were developed. 42,43 Mixed WO 3 /TiO 2 composites were utilized. 27,36,40,[44][45][46][47][48] However, the comparative study among the core-shell and comixed structures, and the effects of structures on reaction mechanisms were not strongly reported. Therefore, we focused on the structural dependence for the dye decomposition.
During the photodecomposition process, adsorption of target compounds is a key rst step to be considered. [49][50][51] In this study, cationic and anionic dyes (MB + and MO − ) were used to analyze the adsorption process of target compounds on the surface of WO 3 -loaded TiO 2 . Then, 3 different types of WO 3 -TiO 2 nanocomposites (a mixture of TiO 2 and WO 3 formed by the sol-gel reaction and core-shell structures of TiO 2 @WO 3 and WO 3 @TiO 2 prepared by a hydrothermal method) were examined as photocatalysts, in addition to the single-component photocatalysts (TiO 2 and WO 3 ). The photodegradation of model target compounds was analyzed in terms of both the adsorption kinetics and reaction mechanism. The results can help guide the further design of photocatalysts consisting of semiconductor nanocomposites.

Sol-gel synthesis of TiO 2 nanoparticles
The TiO 2 nanoparticles were prepared via a sol-gel approach. 52 First, 6.0 mL of TTIP was mixed with 11.6 mL of isopropanol. The mixture was vigorously stirred for 1 h, and 14.6 mL of water was added with vigorous stirring. Aer aging for 24 h, the white precipitate that formed was ltered and thoroughly washed with water. Then, the residue was dried at 80°C for 12 h and calcined at 450°C for 2 h. The obtained white mass was ground into a powder with a mortar and pestle.

Hydrothermal synthesis of TiO 2 nanoparticles
For comparison, TiO 2 nanoparticles were synthesized by a hydrothermal method. 53 First, 5.9 mL of TTIP was dissolved in 9.0 mL of ethylene glycol and stirred for 2 h. Then, the solution was transferred into a Teon-lined autoclave, and 30.3 mL of water was added. The white slurry formed was heated at 220°C for 6 h, and the resulting white precipitate was washed three times with water and twice with ethanol using centrifugation. Then, the white paste was dried at 80°C for 12 h and calcined at 450°C for 2 h. The obtained white mass was ground to a powder with a mortar and pestle. This powder was labeled hyd-TiO 2 .

Synthesis of WO 3 nanoparticles by the sol-gel method
The sol-gel procedure for WO 3 synthesis was adapted from a previous report. 23 A powder of Na 2 WO 4 $2H 2 O (1.0 g) was dissolved in 15 mL of water with stirring. To the solution, 7 mL of 1.0 M HCl was slowly added under vigorous stirring. Then, the obtained light yellowish solution was heated to 80°C for 1 h. Aer the suspension was cooled to ambient temperature, the light yellowish precipitate was separated using centrifugation and washed three times with water to remove residual NaCl and HCl. Then, the yellow paste was dried at 80°C for 12 h and ground into a powder with a mortar and pestle.

Hydrothermal synthesis of WO 3 nanoparticles
WO 3 nanoparticles were also prepared by a hydrothermal method. A powder of Na 2 WO 4 $2H 2 O (2.0 g) was dissolved in 30.0 mL of water. Then, 10 mL of 5 N HNO 3 was added to the solution with vigorous stirring at ambient temperature. The mixture was moved to an autoclave and heated to 220°C for 6 h. Aer the mixture was cooled to room temperature, the precipitates were collected by centrifugation, washed 3 times with water followed by ethanol, and dried at 80°C for 12 h. A yellowish mass was obtained aer calcination at 450°C for 2 h and was ground to a powder with a mortar and pestle. This powder was labeled hyd-WO 3 .
2.6 Sol-gel synthesis of WO 3 /TiO 2 nanocomposites Coprecipitation of WO 3 and TiO 2 was carried out for the preparation of the WO 3 /TiO 2 nanocomposites. First, 6.0 mL of TTIP was mixed with 11.6 mL of isopropanol, and the mixture was vigorously stirred for 1 h. To control the ratio of WO 3 in the nanocomposites, a powder of Na 2 WO 4 $2H 2 O (0.243 g for 5 wt%, 0.401 g for 8 wt%, and 0.512 g for 10 wt%) was dissolved in 14.6 mL of water and added to the TTIP solution with vigorous stirring. Aer the mixture was aged for 24 h, the precipitate was ltered and washed 3 times with water followed by ethyl alcohol. Finally, nanocomposites were obtained aer drying at 80°C for 12 h and calcination at 450°C for 4 h, followed by crushing in a mortar. The single-component metal oxides (TiO 2 and WO 3 ) were denoted sol-TiO 2 and sol-WO 3 , respectively.

Synthesis of core-shell TiO 2 @WO 3
The core-shell nanocomposite TiO 2 @WO 3 was produced with a hydrothermal method. 54 The hydrothermally synthesized TiO 2 (hereaer, called hyd-TiO 2 , 632.5 mg) was dispersed in a solution of Na 2 WO 4 $2H 2 O (100 mg of Na 2 WO 4 $2H 2 O dissolved in 30 mL of water) with stirring for 60 min; the nal mass ratio of TiO 2 : WO 3 = 9 : 1. The resulting white suspension was treated with dropwise additions of 5 N HNO 3 with vigorous stirring. Then, the suspension was transferred to a 50 mL Teon-lined autoclave and heated at 220°C for 6 h. The precipitate was washed using water and ethanol by centrifugation and dried at 80°C for 12 h. Then, the TiO 2 @WO 3 composite was obtained aer calcination in air at 450°C for 2 h, followed by crushing in a mortar.

Synthesis of core-shell WO 3 @TiO 2
The reverse core-shell structure of TiO 2 @WO 3 was also synthesized. First, 2.4 mL of TTIP and 10 mL of ethylene glycol were mixed and stirred for 2 h at ambient temperature. Next, a suspension of WO 3 was prepared by sonicating 70.3 mg of hyd-WO 3 in 28.9 mL of water for 60 min. These solutions and suspensions were mixed in a 50 mL Teon-lined autoclave to make the nal mass ratio of TiO 2 : WO 3 9 : 1. Then, the obtained yellowish to white gel was heated at 220°C for 6 h. Aer cooling, the product was washed using water and ethanol by centrifugation and dried for 12 h at 80°C. Finally, a yellowish to white mass was calcined at 450°C for 2 h, followed by crushing in a mortar.

Characterization
The morphology of the nanocomposites was observed with a eld-emission scanning electron microscope (FESEM, JSM-7900F, JEOL LTD, Japan) at an acceleration voltage of 15 kV. Before SEM inspection, all samples were sputtered with Pt using a JEC-3000FC Auto Fine Coater (JEOL LTD, Japan). Elemental analysis was carried out using an energy dispersive X-ray (EDX) spectrometer equipped with an FESEM. The crystal structures were characterized using XRD (X-ray diffractometer, 2nd Gen D2 PHASER, Bruker) with Cu Ka radiation at an acceleration voltage of 30 kV and a current of 10 mA within the 2q range from 10°to 80°. The presence and oxidation state of each element in the nanocomposites were determined using X-ray photoelectron spectroscopy (XPS, ULVAC PHI 5000 Versa Probe) using Al Ka monochromator (1486.6 eV) X-rays. A UV-VIS spectrophotometer (V-670, JASCO, Japan) was used to measure the absorption spectra of the organic dye solutions. UV-VIS diffuse reectance spectroscopy (DRS) was conducted at a 45°irradiation angle with a UV-VIS spectrometer (SEC2000, ALS, Japan) with a light source from Ocean Optics DH-2000-BAL.

Adsorption analyses
The adsorption properties of the nanocomposites were analyzed by a batch process. A powder of the nanocomposite (4 mg) was dispersed in dye solutions (65 mL) of varying concentrations (1.0, 2.0, 4.0, 6.0, and 8.0 mg L −1 ). Then, the suspension was stirred at ambient temperature under dark conditions for 30 min to achieve adsorption equilibrium. Aer adsorption, the suspension was separated into a supernatant and precipitate (the nanocomposite absorbed some of the dye) by centrifugation (for 60 s at 6000 rpm at neutral pH and room temperature), and the free dye concentration was determined from the UV-VIS absorption spectra of the supernatant: absorbance at the l max of the dye (662 nm for MB + and 464 nm for MO − ) was compared to that of the original dye solution (see Fig. S1 in the ESI †). The quantity of dye adsorbed on each nanocomposite (q e ) in mg g −1 was estimated according to eqn (1).
where C 0 and C e are the dye concentrations (ppm) before and aer adsorption from the solution, V is the volume (L) of the dye solution (=0.065), and m is the mass (g) of the nanocomposite. The theoretical curves were tted to data plots by the soware (ORIGIN 2018) with R 2 values.

Photocatalytic activity analyses
To evaluate photocatalytic activity, a powder of the nanocomposite (4 mg) was dispersed in 65 mL of a dye solution (2.0 ppm) under continuous stirring at room temperature. Aer 30 min in the dark, the dispersion was irradiated at 365 nm using an LED light source (LLS-365, Ocean Optics, Tokyo, Japan). Then, 2.0 mL of the dispersion was sampled at 10 min intervals and centrifuged for solid-liquid separation. The dye concentration of the supernatant was then measured at the l max of the dye. To conrm the reactive species, scavenger solutions (1 ppm of p-BQ, Na 2 -EDTA, and IPA) were used to scavenge superoxide radicals, holes, and hydroxyl radicals, respectively. The quantity (Q d ) of dye degraded was estimated in mg g −1 by subtracting the free dye concentration at time t (C t , mg L −1 ) from the dye concentration before light irradiation. Then, the decomposed quantity of the dye was calculated using eqn (2).
where V is the volume (L) of dye solution at the sampling time, and m is the mass (g) of catalysts.
The decomposition kinetics were analyzed as pseudo rstorder reactions using the Langmuir-Hinshelwood model 55 and were plotted as ln(C 0 /C t ) vs. the photoirradiation time (t, min), as in eqn (4).
where K a is the degradation rate constant (min −1 ). The theoretical curves were tted to data plots by the soware (ORIGIN 2018) with R 2 values.

Structure of nanocomposites
The XRD patterns for the obtained nanocomposites are shown in In the 5 wt% WO 3 / TiO 2 nanocomposite, the presence of WO 3 was not well conrmed, which was probably due to the low concentration of WO 3 .
The 8 wt% and 10 wt% WO 3 /TiO 2 nanocomposites showed diffraction peaks for WO 3 ; however, some peaks (marked by A) signicantly shied from those of sol-WO 3 and were identied as a monoclinic phase of the tungsten oxide W 18 O 49 (JCPDS PDF 05-0392). In contrast, the XRD peaks for TiO 2 in these nanocomposites appeared at the same positions as those in sol-TiO 2 . These results suggested that the W ions were minor components in the nanocomposites (only 8 wt% and 10 wt%) and formed new crystal structures under the inuence of Ti compounds.
The  (422) planes, respectively, which indicated a monoclinic WO 3 crystal (JCPDS PDF 05-0363). Thus, in contrast with the sol-gel method, the hydrothermal synthesis process could be adjusted to result in the monoclinic crystalline phase. Both the hydrothermally produced core-shell TiO 2 @WO 3 and WO 3 @TiO 2 showed diffraction peaks for anatase TiO 2 , and peaks for WO 3 were not observed. The weakness of the XRD peak intensity of WO 3 suggested that the WO 3 shell was very thin and that the WO 3 core was covered with a thick TiO 2 shell.
The crystallite size (D) was computed from Debye-Scherrer's equation (eqn (5)). 52 where K is the Scherrer constant (0.9), l is the wavelength of the X-ray for CuKa (1.54184 Å), b is the FWHM of the peak in radians, and q is the diffraction angle in radians.  2 ), respectively. These results demonstrated that the preparation methods and the mixing ratios altered the crystallite sizes. The hydrothermal process could lead to larger crystals in TiO 2 and WO 3 than the sol-gel process, and a higher content of tungsten could result in larger crystals of TiO 2 .

Optical properties of nanocomposites
The optical properties of the nanomaterials were studied with UV-VIS DRS (Fig. 2). Both sol-TiO 2 and hyd-TiO 2 exhibited high reectance in the range greater than 350 nm, and with the addition of WO 3 , the material reectance decreased in this range ( Fig. 2a and b). The reectance of TiO 2 @WO 3 was slightly lower than that of WO 3 @TiO 2 . Since WO 3 can absorb light of wavelengths shorter than 500 nm, 31,32,56 the lower reectance could be partially explained by the photoexcitation of WO 3 in the short wavelength region. At longer wavelengths (>500 nm), the WO 3 and WO 3 -TiO 2 nanocomposites showed lower reectance than TiO 2 . This could be explained by the existence of WO x (2 < x < 3), which could absorb light, and by the structures of the lm specimens. 32 The bandgap energy (E g ) of the metal oxides was determined using the Kubelka-Munk function and Tauc plots (Fig. 2c and  d). 57 The E g values were evaluated from the intercepts of the energy axis. The E g values of TiO 2 and WO 3 were 3.2-3.3 eV and 2.8-2.9 eV, respectively, and compared with the materials prepared by the sol-gel method, hyd-TiO 2 and hyd-WO 3 showed slightly decreased E g values, which was attributed to their larger crystal sizes. 58 The nanocomposites had E g values between those of TiO 2 and WO 3 : the sol-gel method resulted in E g values of 3.25, 3.20, and 3.18 eV, corresponding to 5 wt%, 8 wt%, and 10 wt% WO 3 /TiO 2 , respectively (Fig. 2c). For core-shell TiO 2 @WO 3 and WO 3 @TiO 2 , the E g values were nearly identical at 3.18 eV (Fig. 2d). The intermediate E g values (in between those of TiO 2 and WO 3 ) suggested interactions between TiO 2 and WO 3 in both the mixed and core-shell structures. Mutual effects in the nanocomposites can decrease the electron-hole recombination rate at the interface for TiO 2 and WO 3 .

Morphological analyses of nanocomposites
The morphologies of the nanocomposites were observed using SEM (Fig. 3). The materials prepared by the sol-gel method consisted of aggregated spherical nanoparticles, and no systematic difference was observed. The hyd-TiO 2 consisted of aggregates of small nanoparticles, while the hyd-WO 3 showed large crystalline structures, as indicated by the XRD analysis. However, the surface morphology of the core-shell nanocomposites did not show a signicant difference. This suggests that the WO 3 shell in the TiO 2 @WO 3 nanocomposite could not develop large crystals because it grew from small TiO 2 particles.
The EDX results identied the presence of titanium (Ti) and oxygen (O) in TiO 2 , tungsten (W) and O in the WO 3 oxide, as well as the presence of Ti, W, and O in the nanocomposites obtained from both the sol-gel and hydrothermal methods. The weight ratios of W to Ti in the sol-gel and core-shell nanocomposites are shown in Table 1. The experimentally obtained W ratios in the nanocomposites prepared by the sol-gel method were signicantly higher than the corresponding theoretical values, which suggests that a signicant amount of Ti compounds were not recovered during the preparation process relative to the W compounds. In contrast, the hydrothermally prepared nanocomposites showed W ratios that agreed well with the theoretical values. This suggests that the hydrothermal process immobilized the Ti compounds in the nanocomposites during crystal growth. The higher W ratio in TiO 2 @WO 3 relative to WO 3 @TiO 2 could be explained by the fact that the TiO 2 core was covered by the WO 3 shell.

Chemical states of nanocomposites
XPS analysis was conducted to determine the chemical states of the sol-gel synthesized and hydrothermally synthesized nanocomposites (Fig. 4). The peak parameters are shown in the ESI (Tables S1-S3 in the ESI †). The survey XPS spectrum showed the existence of oxygen (O 1s), titanium (Ti 2p), tungsten (W 4p and 4f), and carbon (C 1s) on the material surfaces. 44 The carbon contribution originated from the substrate and was used to calibrate the binding energy. To compensate for the charge imbalance in the oxygen-decient state, OH groups were bound to the metal cations. Thus, the density of oxygen vacancies is indicated by the intensity of these mid-binding energy peaks. 61 The third peaks, which were located at the highest binding energy (533.4 eV for 5 wt%, 532.8 eV for 8 wt%, 533.0 eV for 10 wt% WO 3 /TiO 2 , 532.7 eV for TiO 2 @WO 3 , and 532.5 eV for WO 3 @TiO 2 ), could be attributed to contamination from oxygen-containing hydrocarbons, 40 H 2 O, 59 or surface-adsorbed O 2 . 62 The peak areas (%) of the mid-binding energy peaks ranged from 11% to 18%, which indicated that signicant numbers of oxygen vacancies were formed in the nanocomposites, as suggested from the UV-VIS DRS spectra (Fig. 2). These oxygen vacancies could extend the lifetime of the charge carriers and increase the photocatalytic activity of these catalysts.
The presence of only one Ti 2p doublet for Ti 2p 3/2 and Ti 2p 1/ 2 indicated that all Ti atoms shared the same oxidation state (Ti 4+ ). 40 The binding energies of Ti 2p 3/2 for the sol-gel synthesized nanocomposites of 5 wt%, 8 wt%, and 10 wt% WO 3 /TiO 2 were 459.1 eV, 458.4 eV, and 458.9 eV, respectively, while they were 459.3 eV and 458.7 eV for the hydrothermally prepared TiO 2 @WO 3 and WO 3 @TiO 2, respectively. The binding energies of Ti 2p 1/2 for 5 wt%, 8 wt%, and 10 wt% WO 3 /TiO 2 were 464.8 eV, 464.0 eV, and 464.6 eV, respectively, whereas they were 464.9 eV and 464.4 eV for TiO 2 @WO 3 and WO 3 @TiO 2, respectively. The minor change in energy in the nanocomposites could be attributed to the interactions of W-O-Ti bonds; however, the changes were not signicantly or systematic. The peaks of W 4f appeared as two doublets. The rst pair (peaks 1 and 3) might have arisen from W 5+ in substoichiometric WO x (2 < x < 3), 40 which corresponds to an oxygen   @WO 3 , and WO 3 @TiO 2, respectively. These values were larger than those obtained by the EDX method, which suggested that the W component existed more on the surface than in the bulk phase of the nanocomposites. The detection of W in the core-shell WO 3 @TiO 2 nanocomposite and Ti in the core-shell TiO 2 @WO 3 nanocomposite suggested that the core-shell structures were imperfect, although the compositions were controlled to an extent. Fig. 5 shows the ratios of the %area of XPS peaks for each element in the nanocomposites. The changes in the %area of both W 5+ and W 6+ in the nanocomposites indicated the high ratios of W 5+ on the surface of the nanocomposites. The existence of W 5+ can extend the light absorption range, and W 5+ can provide an electron to molecular oxygen to form superoxide radicals (O 2 c − ) under light irradiation. Therefore, a higher ratio of W 5+ on the surface could be an advantage for photocatalysis. On the other hand, the low positive charge on the surface induces low attractive interactions with negatively charged dyes, which is disadvantageous for photocatalysis.

Adsorption process of dyes
The adsorption behaviors of cationic MB + and anionic MO − onto the nanocomposites were investigated in the range of 1-8 mg L −1 initial dye concentrations (the data and the tting curves are shown in Fig. S2 and S3 in the ESI †). The adsorption performances of the pure oxides and composite materials were evaluated by the Langmuir isotherm adsorption models. [63][64][65] The Langmuir adsorption isotherms of the dyes are depicted in eqn (6) below: where q e is the equilibrium adsorption capacity (mg g −1 ) at a specic dye concentration, q m is the adsorption maximum capacity of adsorbents (mg g −1 ) when the concentration of dye is sufficiently high, K L is the Langmuir adsorption constant (L mol −1 ), and C e is the equilibrium free dye concentration (mg L −1 ). The experimental values of q m and K L were tted with the nonlinear tted Langmuir isotherm adsorption curves. The tting equations are given in Table 2.
The adsorption of MB + from different initial concentrations was explored at pH 7.4 without pH control. The adsorption of different MO − initial concentrations was probed at pH 6.7 without pH control. As observed in Fig. 6a, the amount of adsorbed MB + (q e ) rose as the initial concentration increased and became saturated at high concentrations (6-8 ppm). The amount of adsorbed MO − (q e ) also increased as the initial concentration increased in the low concentration range (1-2 ppm) and became almost saturated at higher concentrations (2-8 ppm), except for a slight increase and decline for TiO 2 (Fig. 6b). For the adsorption of MB + , the Langmuir model tted well (R 2 > 0.9), indicating that the adsorption of MB + on the nanocomposites followed a monolayer adsorption process. However, the Langmuir model did not yield good ts for adsorption of MO − (R 2 was in the range of 0.3490 to 0.8428), which could be explained by the low adsorption amounts of MO − (two orders of magnitude smaller than those of MB + , except for TiO 2 ), leading to large errors.
Then, dye adsorption on the nanocomposites was analyzed with eqn (7). 5 where DG is the Gibbs free energy change, R is the gas constant (8.314 J mol −1 K −1 ), and T is the absolute temperature (298 K). The large DG of MB + adsorption could be due to the electrostatic interactions between the metal oxides (negatively charged) and the cationic dye (Table 2). In general, K L and DG increased as the tungsten component increased, which suggested that WO 3 promoted dye adsorption. According to the reported zeta potential measurements of TiO 2 , the surface charge of TiO 2 is positive over the pH range of 2.50 to 7.35, 66 and therefore, the surface of TiO 2 in this study (pH 7.4) was slightly negatively charged. In contrast, the neutral point of WO 3 is pH 1.9, 67 and therefore, a higher proportion of tungsten resulted in a more negative charge on the surface. The difference in the K L and the DG of the core-shell TiO 2 @WO 3 and WO 3 @TiO 2 nanocomposites demonstrated the enhanced adsorption of MB + on WO 3 . However, the nanocomposites provided a higher adsorption capacity than WO 3 , despite their lower interactions with MB + : this could be explained by the large surface area of the nanocomposites. A comparison of preparation methods shows that sol-TiO 2 and sol-WO 3 resulted in stronger interactions (higher K L and DG) but lower adsorption capacities than hyd-TiO 2 and hyd-WO 3 , respectively. As the XRD analysis indicated, the sol-gel method provided smaller crystal grains; however, these crystals were aggregated, and the surface area available for adsorption was likely to be limited ( Fig. 1 and 3). The stronger interaction of the sol-metal oxides with MB + could be due to the defects on the surface of the sol-metal oxides, which served as pockets. Among the nanocomposites, the hydrothermally prepared TiO 2 @WO 3 provided a higher adsorption capacity, although the K L and DG were similar to those for the 5 wt% WO 3 /TiO 2 nanocomposite, which had a lower W-component on its surface (Table 1). Therefore, in terms of availability and efficiency, the W component was more effectively utilized for adsorbing MB + in the nanocomposites prepared by the sol-gel method than those prepared by the hydrothermal method. The advantage of TiO 2 @WO 3 was its large surface area, which allowed for a high adsorption capacity, and this was achieved by the secondary deposition of WO 3 .
In terms of the adsorption behavior of MO − , it should be noted that the DG of MO − adsorption could not be precisely estimated because of the poor correlation coefficients (<0.9) obtained. However, TiO 2 showed a higher adsorption capacity than the others for MO − adsorption, and the nanocomposites also exhibited stronger interactions due to the higher compositional ratio of titanium, as expected from the surface charge of the nanocomposites. The K L and DG of TiO 2 for MO − were comparable to those for MB + adsorption. This nding suggests that the TiO 2 nanoparticles provided binding sites for both the cationic and anionic dyes. The lower adsorption capacity of hyd-TiO 2 for MO − compared with MB + suggests that the number of cationic binding sites was lower in hyd-TiO 2 , which could be due to the difference in crystallinity and the crystal structures shown by the XRD measurements (Fig. 1). The tungsten enhanced the adsorption of MB + and weakened the adsorption of MO − on the nanocomposite surfaces. 68 However, the K L and DG values indicated that some nanocomposites also provided effective binding sites for MO − , especially the 5 wt% WO 3 /TiO 2 and WO 3 @TiO 2 nanocomposites.

Photocatalysis of dyes
The photocatalytic activity of the synthesized materials was evaluated by dye decomposition. The resulting absorption spectra are shown in Fig. S4 and S5 in the ESI. † The decomposition curves and their kinetic analyses for MB + and MO − are shown in Fig. 6.
The MB + was irradiated with UV light in an aqueous solution of pH 7.4. Measurements were also performed under dark conditions, and the decrease in concentration was in the range of 1-8% of the initial concentration, which could have been caused by the disaggregation of the nanocomposites induced by stirring. In the absence of nanocomposite materials, the degradation of MB + was 14.4% under UV irradiation, which was lower than that in the presence of nanocomposites. Compared with TiO 2 and WO 3 , the nanocomposites exhibited better photocatalytic activity under UV light irradiation. The photocatalytic activity of 8 wt% WO 3 /TiO 2 was highest among the solgel nanocomposites, with 94.9% decomposition aer 2 h, while The reaction rates were analyzed as pseudo rst-order reactions using eqn (4) and the Langmuir-Hinshelwood model. 65 The rate constant ðK * a Þ for each nanocomposite and dye is summarized in Table 3.
These activities were inconsistent with the orders of both the adsorption rate (K L ) and the adsorption capacity (q m ) of the nanocomposites (Table 2). To some extent, the magnitude of the reaction rates could be explained by several factors: (1) the tungsten in the nanocomposites provided higher reaction rates due to its strong interaction with MB + , (2) the absorbance of TiO 2 was low at an excitation wavelength of 365 nm, (3) among the sol-gel nanocomposites, the adsorption capacities determined the order of the reaction rates, and (4) TiO 2 @WO 3 showed highest rate constant, which could because it also had the highest adsorption capacity. However, these explanations were not sufficient for explaining the lower rate constant of WO 3 , which had a high adsorption constant and moderate adsorption capacity. Upon dividing K * a by K L and q m , WO 3 exhibited the lowest rate constant per (adsorption rate x adsorption mass), while that of TiO 2 @WO 3 was the highest, followed by that of hyd-TiO 2 . Therefore, the photocatalytic activity was not determined only by the adsorption amount and the adsorption rate.  For comparison, the anionic MO − was also degraded at pH 6.7 under UV light irradiation. The reaction rate was much lower than that of MB + , which could be expected from the adsorption parameters discussed above ( Table 2). Among the nanocomposites, WO 3 @TiO 2 demonstrated the highest efficiency (degradation of ∼6% MO − ), whereas TiO 2 @WO 3 demonstrated the lowest efficiency (degradation of ∼2% MO − ). The negative surface charge from the tungsten oxides had adverse effects on the photocatalysis of MO − . Upon dividing K * a by K L and q m , the nanocomposites demonstrated a clear tendency: a higher composition of tungsten resulted in a higher rate constant. This could be explained by the stronger light absorption at 365 nm by tungsten components. However, the higher rate constants per (adsorption rate × adsorption mass) of 10 wt% WO 3 /TiO 2 and TiO 2 @WO 3 suggested that the nanocomposite decomposed MO − more effectively than TiO 2 and WO 3 .
Compared with other studies for the MB + degradation performance (Table S4 in the ESI †), the activity of 8 wt% WO 3 / TiO 2 was 1.34 times higher than that of 25 wt% mixed WO 3 / TiO 2 . 46 The activity of the core-shell TiO 2 @WO 3 of the current study was 3.59 times higher than that of 36 wt% core-shell WO 3 /TiO 2 . 43 These results indicate that the large amount of WO 3 is not essential for the effective photocatalyst.

Dye decomposition mechanism
To study the active species present during photocatalysis with the nanocomposites, dye photocatalysis was analyzed in the presence of active species scavengers. The active species generated by TiO 2 and WO 3 are considered to be superoxide anions (O 2 − ), holes (h + ), and hydroxyl radicals (HOc), which can be scavenged by p-BQ, Na 2 -EDTA, and IPA, respectively. 38,48,69,70 The degradation curves and the kinetic analyses are shown in Fig. 7 and Table 4.
The addition of scavengers decreased the decomposition rate in all cases. These results indicated that hydroxyl radicals were the most active species for all the photocatalysts examined. When IPA was used to quench hydroxyl radicals, the K a values of the photocatalysts decreased to a similar level (0.70-0.99 × 10 −2 min −1 ). The activity of TiO 2 @WO 3 exhibited the greatest decrease with the use of IPA, and the effect of IPA on WO 3 @TiO 2 was the smallest. This suggests that WO 3 on the nanocomposite surface mainly provides hydroxyl radicals as the active species; the OH − groups attached to W 5+/6+ could be directly oxidized by holes to generate hydroxyl radicals.
A comparison of the effects of p-BQ and Na 2 -EDTA show that the K a of TiO 2 @WO 3 decreased more with the addition of Na 2 -EDTA than p-BQ, while the 8 wt% WO 3 /TiO 2 nanocomposite was similarly quenched by both scavengers. These results suggest that the activity of the core-shell TiO 2 @WO 3 depended on the activity of holes more than photoexcited electrons, while the codeposited WO 3 /TiO 2 used both to a similar extent. In the reversed structure, the core-shell WO 3 @TiO 2 nanocomposite also demonstrated a stronger effect with Na 2 -EDTA than p-BQ; however, its activity was much lower than that of TiO 2 @WO 3,